Conventional microwave ovens, though possessing many advantages, suffer from an inability to heat items placed within them evenly at all points on their surface. The unevenness of the microwave oven prepared comestible is in part the result of the unevenness of the incident microwave energy. This problem can be circumvented by varying the cooking process or by incorporating an energy moderator. By varying the cooking process to periodically reposition the article being cooked, cooking evenness can be improved. However, varying the process inevitably means that greater attention is required. Thus a variety of moderators have been proposed to avoid the requirement of attention to the cooking comestible.
One approach has been to provide a mechanism which automatically repositions the food item within the microwave energy field. Rotating shelves, for example, that described in U.S. Pat. No. 3,428,773 issued Feb. 18, 1969 to Waldenfels, have been introduced to lessen the effects of nonuniform fields of microwave energy in microwave ovens. In a converse approach, the food is kept stationary and the field is "moved" or "stirred". Rotating mode stirrers, such as that disclosed in U.S. Pat. No. 3,819,900 issued June 15, 1974 to Ironfield, have been provided to lessen the non-uniformity of the field of microwave energy in microwave ovens.
Another approach has been to partially or selectively shield the item being cooked with a specially designed food container. For example, U.S. Pat. No. 3,547,661 which issued Dec. 15, 1970 to P. N. Stevenson discloses a container and food heating method where apertures of various sizes are provided on the top and bottom in registered relation. Such apertures may also be partially masked by microwave reflective material as indicated in FIGS. 1 and 3, areas 25-28. Various sizes of apertures or of partial masking ostensibly provide means for selectively heating different items at different temperatures simultaneously. U.S. Pat. No. 4,013,798 which issued Mar. 22, 1977 to Costase also discloses a selectively shielded microwave cooking structure comprising registered openings of various sizes. The use of apertures of various sizes and shapes in the top of a microwave cooking food tray which is otherwise microwave reflective is disclosed in U.S. Pat. No. 3,672,916 which issued June 27, 1972 to H. J. Vernig and U.S. Pat. No. 3,219,460 which issued Nov. 23, 1965 to E. Brown.
The prior art also includes means in the form of a cooking container for moderating the incoming microwave energy. For example, U.S. Pat. No. 4,144,438 issued on Mar. 13, 1979 to Gelman describes a microwave energy moderating bag with a foil lamina 23 perforated by an array of apertures 30 which are sufficiently large and numerous to render the bag 20 substantially transparent to microwave energy of a predetermined frequency. However, the apertures are sufficiently small that such microwave energy which passes into the bag in a microwave oven will be sufficiently moderated to precipitate uniform cooking of a foodstuff disposed therein.
Water has been used in the past to improve the evenness of the cooked foodstuff. For example, Soviet Pat. No. 501,748 issued Feb. 5, 1976 discloses a cooking bag for meat or fish which involves surrounding the food with two closed, unvented cellophane bags having water between the two bags in an amount up to 15% of the weight of the food. The food is cooked by a series of 2 to 3 minute heating cycles separated by 2 to 3 minute cooling cycles. A similar approach is to convey the food item through a water bath where it is exposed to microwave energy. Examples of such devices are disclosed in German Offenlegungsschrift Pat. No. 2,704,563 issued Aug. 25, 1977 and U.S. Pat. No. 3,809,845 issued May 7, 1974 to Stenstrom. Stenstrom discloses a thick water layer (2.5 mm.) above and below the food portion. In addition, it is known that the Litton Company of Minneapolis, Minn. is currently marketing a device known as a "Simmer Pot" which is a porous clay pot with a water absorbent clay lid which is soaked in water for 30 minutes to overnight. After soaking the lid, the food item is placed within the closed clay pot and cooked. A similar device is marketed by El Camino Products, Inc. of Penoga Park, California under the brandname Olde World Roaster.
The prior efforts to make the cooking of food items within a microwave oven more uniform are subject to a variety of shortcomings. Devices which require oven redesign are of little use to current oven owners. Devices using metallic elements are prone to arcing problems which must be avoided with concommitant increase in the cost of the product. Devices using water baths are not suitable for use in the home, and bags with water requiring short cooking cycles or long soaking cycles are inconvenient. Most importantly the prior practice in this field is subject to improvement in terms of the extent of evenness of the cooked comestible which is accomplished.